1. Field of the Invention
The present invention relates to a semiconductor switching device provided with a weak-current detecting function.
2. Description of the Related Art
It may be shown as a conventional semiconductor switching device with a current detection function in FIG. 1. This switching device is installed between a power 101 and a load 102. This device has the structure which connected a shunt resistor RS with the switching device which consists of power FETs, etc. in series. Usual the shunt resistor and the switching device are included in the identical substrate. The current detection is carried out using the potential difference which arises in shunt resistor double end. The potential difference arises in the double end of shunt resistor RS, when load current flows. This potential difference is amplified in a differential amplifier 911 and a direct current amplifier 913. The amplified potential difference by going through an A/D converter 902, it is read in the microcomputer, and the microcomputer judges overcurrents and less currents, etc., Negligible currents such as the leakage current can be also detected in theory by this method. Since the value of the detection current is small, it becomes a problem that the detection sensitivity is raised. In the reason, the countermeasure of raising the amplification factor of the direct current amplifier 913 and the differential amplifier 911 and of raising the value of the shunt resistor is required. It becomes a problem that the exothermic reaction of the shunt resistor as a load current flowed increases, when the shunt resistor is increased. In addition, there is a problem that the voltage supplied for the load lowers, when the voltage drop which arises in the shunt resistor increases. In the meantime, there is a problem that the detection accuracy becomes bad, since an S/N ratio deteriorates, when an amplification factor of the differential amplifier 911 and direct current amplifier 913 is raised. In method using this shunt resistor, when the value of detecting negligible current decreases, this problem becomes difficult and the accomplishing goal becomes difficult.
An original function of the semiconductor switching device is to supply the electric power from the power source 101 in the load 102. Except for it, it has the function, namely the overcurrent protection function, when the trouble in which the wiring between the load 102 and the shunt resistor RS touched ground arose, which prevents that a large current flows for the wiring with the shunt resistor. It has the function which prevents that large current flows for the wiring with the shunt resistor. This overcurrent protection function becomes a essential, when the leakage current from the power line is measured using the shunt resistor. There is a case in which the semiconductor switching device contains heating and cutting off function. The semiconductor switching device contains a power element (main FET) QM, a resistor RG, a temperature sensor 121, a latch 122 and an overheat breaking element FETQS, as it is shown in FIG. 2. It has heating and cutting off function which compulsorily turns off temperature sensor built-in FETQF by containing gate interception circuit, when the junction temperature of FETQF rises until exceeding the regulation value. Namely, if the temperature sensor 121 detects that the temperature of the power element QM increases above the predetermined value, the latch 122 holds the increased temperature information to turn on the breaking element QS, which forcibly turns off the power element QM. The temperature sensor 121 consists of four diodes that are connected in series, are made of, for example, polysilicon, and are integrated in the vicinity of the power element QM. As the temperature of the power element QM increases, a forward voltage-drop of the four diodes of the temperature sensor 121 decreases. The gate potential of an FET Q51 drops to low, the FET Q51 changes from ON to OFF. This pulls up the gate potential of an FET Q54 to the potential of a gate control terminal G of the element QF, to change the FET Q54 from OFF to ON. As a result, the latch 122 latches "1" to provide an output of high. This output changes the breaking elements QS from OFF to ON to short-circuit the true gate TG and source S of the power element QM. Consequently, the power element QM changes from ON to OFF. Namely, the power element QM is turned off.
In FIG. 1, a zener diode ZD1 keeps a voltage of 12 V between the gate terminal G and source terminal S of the element QF and serves as a bypass for an overvoltage so that the overvoltage may avoid the true gate TG of the power element QM. The driver 901 has differential amplifiers 911 and 913 serving as a current monitor circuit, a differential amplifier 912 serving as a current limit circuit, a charge pump 915, and a driver 914. According to an ON/OFF control signal from the microcomputer 903 and an overcurrent signal from the current limit circuit, the driver 914 drives the true gate TG of the element QF. If an overcurrent exceeding an upper limit is detected through the differential amplifier 912 due to a voltage drop at the shunt resistor RS, the driver 914 makes the element QF nonconductive. If the current again decreases below the upper limit, the driver 914 makes the element QF conductive. On the other hand, the microcomputer 903 always monitors a current through the current monitor circuit made of the differential amplifiers 911 and 913, and if detects an abnormal current exceeding a normal value, makes the driver 914 turn off the element QF. If the temperature of the element QF exceeds a predetermined value before the microcomputer 903 issues an OFF instruction to the driver 914, the overheat breaking function mentioned above turns off the element QF.
The prior art must employ the shunt resistor RS connected to a power line in series, to detect a current in the power line. When a current supplied from a power source to a load through the power line is large, the shunt resistor RS causes a large heat loss that is unignorable.
To detect a weak current in the power line, the resistance value of the shunt resistor RS must be large. This, however, produces a large amount of heat when a large current is passed through the power line. To avoid this, the resistance value of the shunt resistor RS must be decreased, and then, it becomes difficult to detect a weak current in the power line.
The prior art detects a current in the power line according to a voltage drop at the shunt resistor RS. To achieve this, the prior art must have a current monitor circuit involving the shunt resistor RS, A/D converter 902, microcomputer 903, etc. These parts need a large space and are expensive, thereby increasing the size and cost of the switching device.
Even if the load controlled by the switching device is OFF, a leak current will flow if the power line between the load and the power source causes a grounding fault or a short circuit due to, for example, abrasion, moisture, or corrosion. If no measures are taken, the leak current will increase to cause a fire. It is strongly required, therefore, to monitor a weak leak current while the load is OFF.